PLCs and Fuzzy Logic (7)


Fuzzy logic can be applied to a wide array of applications and industries, given that the application requires reasoned output values derived from system inputs or feedback.

Figure 10 shows an example of a typical fuzzy logic application, a two-conveyor packaging system. The objective of this application is to synchronize two conveyors so that parts and packaging boxes are positioned correctly, regardless of the part and packaging box positions and the speed of conveyor.

System Description and Operation
In the system, the parts travel on conveyor A, pass onto the connecting conveyor, and then go to
conveyor B, where they are boxed before going to the wrapping machine. The photoelectric sensors PE1 and PE2 detect the presence of a part and initiate a count to determine the part’s position from encoder 1. PE3 and PE4 detect the presence of a box and determine its position based on the count inputs from encoder 2.
The control objective is to adjust the speed of conveyor B so that the packaging boxes arrive at the same time as the parts, meaning that they meet at the connecting conveyor. The process information required to implement this control is:
• the offset between the part and the packaging box
• the rate of change of the offset

The parts on conveyor A travel at random intervals, but they travel at a constant speed. The boxes on conveyor B occur at regular intervals, and the speed of conveyor B can be controlled. The photoelectric sensors will be used in the PLC program to detect when to start timing and computing the data from the encoders. The two fuzzy input variables are the part/box offset and the rate of change of the offset.
If a box is present at PE3 and a part is present at PE1, conveyor B should run at the same speed as conveyor A (the reference speed set initially by the operator). If the box is at PE3 but the part is behind PE1, the system will slow conveyor B until the part is at PE1. At this time, the fuzzy controller will indicate an increase in the speed of conveyor B so that it will catch up with conveyor A. The distance traveled by the box is calculated, using the input data from encoder 2, as the difference between the time the box passes PE3 and the time the part passes PE1. The difference in counts between encoder 1 and encoder 2 provides the part/box offset data. This value, denoted as X, is calculated as:

X = (Encoder 1 counts) – (Encoder 2 counts)

The rate of change of the offset is calculated as the difference between the current offset reading n) and the previous one (X(n–1)):

to be continued…………….

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